The practice of optimizing images is mainly one of observing the materials and the on screen scan, and making a comparison. The goal of the comparison is to retain as much visual information about the materials being scanned as possible.
things to look to when making the visual assessment.

are the light areas and the dark areas retaining their details in the on screen scan. (if not there is a problem with brightness)

are there grey tones that act as a"bridge" between light and dark tones. (if not there is a problem with contrast)

are the lightest areas a "flat" white tone or is there some hint of volume, detail, or gradient within that light area. (bringing down the level of contrast, or bringing down the brightness can reveal these details.

Focus

The image below illustrates the key point to take into consideration when assessing proper focus.

Gamut

Out of gamut and banding problems can occur when the level of contrast has been lowered to much

Tone

The image below illustrates several states an image can be in with the center image being the best.